This invention is directed generally to the field of conductivity measurement and more particularly to a novel and improved apparatus and method for measuring the electrical conductivity of a subject by measuring the conductivity of an electromagnetic field producing element or elements, with the subject of interest being placed in the electromagnetic field produced thereby.
The prior art has proposed a number of methods and apparatus for determining the electrical conductivity of a subject by field disturbance and measurement techniques. For example, Harker, U.S. Pat. No. 3,735,247 is directed to apparatus for measuring the fat-to-lean ratio of meat by placing a package or sample of the meat in an electromagnetic field generated by a solenoidal coil and measuring the load on a drive source for the coil. Similarly, International Patent Application No. PCT/US 81/00602, published as International Publication No. WO 81/03226 is directed to the generation of pictorial images of selected volume elements of materials by a tomographic technique. In this technique, the material is brought within the influence of a relatively low-strength electromagnetic field and subjected to plural preselected frequencies to produce output data which is used to generate an image reflective of the conductance properties of the selected volume elements.
The prior application of David B. Funk et al., Ser. No. 375,552, now U.S. Pat. No. 4,496,907, filed on May 6, 1982 discloses apparatus for non-destructively determining the ingredients of a sample and specifically for determining the fat-to-lean ratio in a sample of a meat product. This application discloses a field producing coil, the sample of interest being placed entirely within the field of this coil, and the electrical conductivity of the coil being measured both with the sample absent and the sample present within the coil. A microcomputer is then utilized to determine the fat-to-lean ratio of the sample, based upon impedance or conductivity measurements taken of the coil.
The present invention is directed particularly to the problem of determining the conductivity of a relatively large subject, such as a live human or large live animal subject. Hence, unlike the prior art patents directed to the measurement of fat and lean in a sample of meat, the subjects to which the present application is directed are relatively non-uniform in composition and do not lend themselves to physical isolation of relatively smaller representative samples for testing. Moreover, these live human and animal subjects will be non-uniform in size, dimensions, and composition and distribution of components from one to the next, and hence cannot be placed in a field for test purposes in a uniform fashion from subject to subject. Also, the subjects may move during testing, unlike the inanimate subjects addressed in some of the above-referenced patents.
I have discovered that relatively simple conductivity measurements taken in a relatively low energy magnetic field may be subjected to computer aided analysis techniques to determine much useful information concerning the composition of the subject and/or of various parts of the subject. Such relatively low energy testing may advantageously be carried out without any specialized site preparation, shielding, or the like and without fear of harm to the subject. In contrast, many high-energy techniques such as X-ray, nuclear magnetic resonance (NMR), and other similar techniques require highly specialized room or site preparation and/or shielding and may also pose some exposure risks to both subjects and equipment operators.
A number of problems have arisen with respect to the testing of human or large animal subjects by the above-mentioned low-energy magnetic field disturbance technique. For example, the prior art has assumed that the subject must be placed entirely within a uniform and continuous field to assure accurate and repeatable results. In this regard, it has been believed that field disturbances and other areas of non-uniformity, such as boundary conditions, can result in unpredictable variations in test results. Accordingly, it has been believed that the axial length of a magnetic field used for testing (and hence of a coil for producing this field) must be on the order of two times the length of the subject and that the radius thereof must also be considerably greater than the greatest transverse dimension of the subject.
This relatively long axial length is to assure that the subject, when centered in the field, remains relatively far removed from axial end boundary effects of the field experienced at ends of the field-producing element such as a coil. Similarly, the relatively large radius is to permit the subject to be placed substantialy centrally within the field in the radial direction, to avoid non-uniform boundary effects experienced in the regions relatively close to the inner surface of the field-producing coil.
In this regard, a cylindrical coil has been utilized. One such coil previously utilized was of such a size that a rectangular housing or enclosure therefor was on the order of 12 feet in length, 6 feet in height, and 5 feet in width. Such a size requirement makes such an apparatus difficult to construct and transport and erect on the site at which testing is to be carried out. Moreover, such an apparatus requires a relatively large room or area for testing and cannot be readily moved or transported to another room or area to carry out testing. Hence, it has been believed heretofore that the size required of such an apparatus limited its portability and usefulness.
A second problem which has arisen with the techniques and apparatus utilized in some of the foregoing patents is the relatively small amount of data or information obtained by taking but a single reading of the subject or sample placed entirely within the field producing coil. The above-referenced International Publication has suggested utilizing a plurality of preselected frequencies to provide additional output data, and in addition suggests providing relative movement between the sample and the field. However, as previously mentioned, it was also believed that the sample should be maintained entirely within a uniform region of a field to avoid unpredictable effects of non-uniform boundary conditions at edges of the field producing coil or other apparatus. Hence, with respect to the human testing apparatus, satisfying both requirements of moving the sample relative to the field and of maintaining the sample in a uniform portion of the field would require an even larger apparatus than mentioned above. Moreover, the use of multiple frequencies in apparatus for human testing raises a number of problems, in that each frequency utilized must be approved for such use by the Food and Drug Administration of the United States Government. Obtaining such approval can be a relatively complex and expensive procedure. Hence, it is preferable to minimize the frequencies utilized and preferably to utilize but a single frequency for measurement.
A third problem which has arisen is that of the stability of the detector apparatus. In this regard, the electromagnetic field generating element or coil, the drive circuit for this coil and the measuring or detecting circuit for measuring the conductivity or other electrical properties of the coil are subject to unpredictable changes in gain, response and the like. Such changes may occur both from measurement to measurement over a period of time and from instrument to instrument, that is, among a plurality of otherwise identically constructed test instruments. In this regard, the conductivity or other electrical properties of a given coil may change from time to time due to interference from various factors such as environmental changes, aging of components, and the like. Similar factors may also cause variations in the gain or other electrical properties of the circuits used to drive the coil or other field producing element. Similarly, the measurement or detection circuits will include a number of electrical and electronic circuit elements which may similarly vary from time to time with environmental effects, aging or the like, in gain, scaling factors, response and the like. Hence, it has heretofore been a difficult problem to assure stability of the detector and correspondent accuracy and normalization or repeatability of readings. The same factors also make standardization of readings from one instrument to another difficult to obtain. Such standardization and stability are important in assuring statistical validity of the results obtained over a period of time on a number of subjects and with a number of instruments.